Signal enhancement by trapping in microscale liquid chromatography: numerical modelling

We report on a series of computer simulations that have been made to explore the operation and performance limits of a periodically heated and cooled trapping segment (liquid analog of the modulator device used in GC×GC) interfacing the separation column and the detector. The initial peak width and the retention of the molecules on the trapping segment have the most important influence on the trapping efficiency. Higher retention of the trapping segment and smaller peaks will lead to higher signal enhancements. However when the resulting concentration gradients become too large, as is the case for very small peaks and/or for very high retention factors on the trapping segment, the dispersion will strongly decrease the focussing efficiency. The molar diffusion coefficient and the linear velocity mainly have an impact on the dispersive behaviour, which can be directly calculated from the associated plate height values. General design rules for the trapping segment, validated with the computer simulations, give a good estimate of the required trapping time and the length of the trapping segment. Equations for the estimation of the trapped peak width and signal enhancement are also given.